Barbaros Celikkol

A case study of a modified gravity type cage and mooring system using numerical and physical models

A modified gravity-type cage, developed by SADCO Shelf Ltd., was examined using numerical and physical models to determine if the cage and mooring system is suitable for an exposed site south of the Isles of Shoals, NH. The 3000-m/sup 3/ SADCO Shelf Submersible Fish Cage has angled stays between the upper framework and the ballasted bottom rim (in addition to net) to resist the horizontal shear deformation. The mooring system consists of three legs-each made up of a taut vertical chain and an angled rope, both leading to deadweight anchors. Normalized response amplitudes (response amplitude operators) were found for motion response in heave, surge and pitch, and load response in the anchor and bridle lines, in regular (single frequency) waves. In addition, a stochastic approach was taken to determine the motion and load transfer functions in random waves using a spectrum representative of seas at the selected site. In general, the system motion had a highly damped response, with no resonant peaks within the wave excitation range of 0.05 to 0.45 Hz. The anchor line force response was at all frequencies below 5 kN per meter of wave amplitude. The physical model tests showed consistently more conservative (larger) results compared to those for the numerical model.

Open ocean aquaculture engineering : Mooring & net pen deployment

An open ocean aquaculture net pen system was developed for offshore deployment south of the Isles of Shoals, New Hampshire, USA in 55 meters of water. Two cages were specified for the growout of summer flounder as part of an interdisciplinary effort at the University of New Hampshire involving engineers, biologists, economists, and commercial fishermen. This effort included the design of mooring system suitable for the offshore environment. Assembly and deployment of these cages and associated moorings occurred in the summer of 1999. An overview of the procedures and techniques used during these efforts are presented here.

Marine sediment acoustic measurement system

Abstract This paper describes a system that has been developed to measure compressional wave speed in cored marine sediments onboard ship. The structure enables one to secure an extruded core sample to its base and to move acoustic probes to a desired location, implant them to a specified depth into the sample and perform the measurement. The acoustic measurement system is a pulse-time delay system measuring time difference over a fixed path length and the temperature of the sediment. The time difference and temperature measurement systems are comprised of task oriented components and are housed in a single portable box. The system is adaptable to the various sample sizes obtained with the coring apparatus presently in use. Initial field tests indicate that ship motion has no effect on the system. Data collected from cores has been classified according to sediment type and displays good agreement with data presented by Hamilton (1970). The difference in compressional wave velocity, based on sediment type, for the two studies is ⩽ 5 m per sec.

Application of a Finite Element Hydrodynamic Model to the Great bay Estuary System, New Hampshire, U.S.A.

Abstract A vertically integrated, one layer, hydrodynamic model developed by Connor and Wang for Massachusetts Bay has been adapted to the Great Bay Estuary, New Hampshire. This finite element model was found to be well suited to the complexities of the estuary. Initial model development and testing was carried out. The bottom friction coefficient was the dominant parameter in the calibration process. A method for selection of the bottom friction coefficient in the calibration process was developed. Eddy viscosity terms, which primarily act as numerical damping, were included. The model was calibrated using an extensive set of tidal sea level and current data recently collected by the University of New Hampshire and the National Ocean Survey (NOS). The data sets used in the calibration and validation process include sea level data used to specify open boundary conditions in the model. Validation results show good agreement between tidal elevation measurements and model predictions. Predicted currents have a different scale of resolution than the current measurements, making direct comparisons difficult to interpret. A program for processing current data to make more meaningful comparisons is presented briefly.

Position, Orientation and Velocity Detection of Unmanned Underwater Vehicles (UUVs) Using an Optical Detector Array

This paper presents a proof-of-concept optical detector array sensor system to be used in Unmanned Underwater Vehicle (UUV) navigation. The performance of the developed optical detector array was evaluated for its capability to estimate the position, orientation and forward velocity of UUVs with respect to a light source fixed in underwater. The evaluations were conducted through Monte Carlo simulations and empirical tests under a variety of motion configurations. Monte Carlo simulations also evaluated the system total propagated uncertainty (TPU) by taking into account variations in the water column turbidity, temperature and hardware noise that may degrade the system performance. Empirical tests were conducted to estimate UUV position and velocity during its navigation to a light beacon. Monte Carlo simulation and empirical results support the use of the detector array system for optics based position feedback for UUV positioning applications.

Assessment of a submerged grid mooring in the Gulf of Maine

The University of New Hampshire (UNH) developed and maintained an offshore aquaculture test site in the Western Gulf of Maine, south of the Isles of Shoals in approximately 50 m of water. This site was designed to have a permanent moored grid to which prototype fish cages or surface buoys could be attached for testing new designs and the viability of the structure in the exposed Gulf of Maine. In 1999, the first moorings deployed consisted of twin single bay grids each capable of each securing one fish cage. These systems were maintained until 2003. To expand the biomass capacity of the site, the single bay moorings were recovered and a new four bay submerged grid mooring was deployed within the same foot print of the previous twin systems. This unique system operated as a working platform to test various structures, including surface and submersible fish cages, feeding buoys and other supporting equipment. In addition, the expanded capability allowed aquaculture fish studies to be conducted along with engineering and new cage/feeder testing. The 4 bays of the mooring system were located 15 meters below the surface. These bays were supported by nine flotation elements. The system was secured to the seafloor on the sides with twelve catenary mooring legs, consisting of Polysteel® line, 27.5 m of 52 mm chain and a 1 ton embedment anchor, and in the center, with a single vertical line to a 2 ton weight. To size the mooring gear, the UNH software package Aqua-FE was employed. This program can apply waves and currents to oceanic structures, predicting system motions and mooring component tensions. The submerged grid was designed to withstand 9 meter, 8.8 second waves with a 1 m/s collinear current, when securing four fish cages. During its seven year deployment, the site regularly experienced extreme weather events, most notably a storm with a 9 m significant wave height, 10 second dominate period in April 2007. The maximum currents at the site were observed during internal solitary wave events when 0.75 m/s currents with 25 minute periods and 8 m duration were observed. The mooring was recovered in 2010 after 7 years of continuous deployment without problems. The dominate maintenance requirement of the mooring was the cleaning once a year of excessive mussel growth on the flotation elements and grid lines. No problems of anchor dragging or failure of mooring components were documented during the deployment. Upon recovery, critical mooring components were inspected and documented, focusing on items with wear or other areas of interest. The mooring proved to be a reliable, stable working platform for a variety of prototype ocean projects, highlighting the importance of a sound engineering approach taken in the design process.

Practical Applications of Numerical Modeling using Aqua-FE: A Case Study

A numerical model of an American Soybean Association (ASA) cage system was constructed using a finite element program developed at the University of New Hampshire (UNH) called Aqua-FE. The small volume, high density aquaculture system was modeled to determine how the system will operate in normal and extreme environmental conditions. The goals of the study were to determine the maximum loads in the system during tropical storm conditions and determine a similar cage systems response under specified environmental criteria. The cage is currently deployed in Weitou Bay, China. The system consists of a 100 m3 cage (2 m times 4.5 m times 7 m) secured in a single point mooring. The rigid HDPE cage is held to the mooring by two sets of bridle lines, attached to the upper and lower cage framework. Chain ballast hangs below the lower cage rim providing a restoring force. A deadweight anchor secures the system to the seafloor. A 90 kg float suspends the single point mooring and serves as a tie-up location for servicing vessels. Aqua-FE can apply wave and current loading on truss and buoy elements by utilizing the Morrison equation adopted for analysis of aquaculture net pen systems. The algorithm employs a nonlinear Lagrangian formulation to account for large displacements of structural elements. In addition, the unconditionally stable Newmark direct integration scheme is adopted to solve the nonlinear equations of motion. Hydrodynamic forces on the structural elements are calculated using the Morison equation modified to account for relative motion between the structural element and the surrounding fluid. Maximum loads in the mooring gear approached 56 kN during the storm events. When various current velocities were applied, the cage submerged to a maximum depth of 16.4 meters

Shoal formation in the Piscataqua River, New Hampshire

The formation of a shoal was investigated in the Piscataqua River, New Hampshire, which is a well-mixed channel with low freshwater flow and tidal currents up to 2.3 m s−1. Observations of sediment characteristics, bathymetry, and bottom current were made, and theory was used to predict bedload transport. Sediment sampling showed the bottom material to be coarse sand and gravel, and sidescan sonar revealed large sand waves directed upriver at the shoal. Bottom current measurements were made along transects upriver and downriver of the shoal and downriver of an adjacent deepwater area that was also studied for comparison. Bedload flux inferred from current measurements using the Brown-Einstein theory indicated that transport is generally directed upriver. Sediment budget calculations showed the shoal area to be depositional before, immediately after, and subsequent to a dredging operation at rates of 0.36 m yr−1, 1.06 m yr−1, and 0.35 m yr−1, respectively. Predredge and subsequent rates were consistent with the historical record of removal by dredging at the shoal.

Optical Detector Array Design for Navigational Feedback Between Unmanned Underwater Vehicles (UUVs)

Designs for an optical sensor detector array for use in autonomous control of unmanned underwater vehicles (UUVs), or between UUVs and docking station, are studied in this paper. Here, various optical detector arrays are designed for the purpose of determining and distinguishing relative 5 degrees-of-freedom (DOF) motion between UUVs: 3-DOF translation and 2-DOF rotation (pitch and yaw). In this paper, a numerically based simulator is developed to evaluate varying detector array designs. The simulator includes a single light source as a guiding beacon for a variety of UUV motion types. The output images of the light field intersecting the detector array are calculated based on detector hardware characteristics, the optical properties of water, and expected noise sources. Using the simulator, the performance of planar and curved detector array designs (of varying size arrays) are analytically compared and evaluated. Output images are validated using empirical in situ measurements conducted in underwater facilities at the University of New Hampshire, Durham, NH, USA. Results of this study show that the optical detector array is able to distinguish relative 5-DOF motion with respect to the simulator light source. Furthermore, tests confirm that the proposed detector array design is able to distinguish positional changes of 0.2 m and rotational changes of 10 ° within 4-8 m range in x-axis based on given output images.

Distance detection of Unmanned Underwater Vehicles by utilizing optical sensor feedback in a leader-follower formation

This paper proposes an optical detection system between a leader and a follower Unmanned Underwater Vehicle, specifically Remotely Operated Vehicles (ROVs). Cost efficient photodetectors and a single LED light source are used to develop distance detection algorithms to detect translational motion in x-and y-axis directions. Analytical simulations are performed where light is modeled as a first order Gaussian function and integrated into the nonlinear ROV dynamics. The stability of a proportional derivative (PD) controller is shown via Lyapunov stability, as in Fossen [7]. Both leader and follower ROV motions are simulated and experimental results from the distance detection algorithm are shown for proof of concept. In this stage of research, all experiments are performed out of water. Initial results indicate that the proposed detection system shows promise as a precursor stage to underwater testing.